I occasionally use the killall command to kill processes. The reason why I say ocassionally is that in some cases it hasn't worked for me.

A recent example was with thunderbird where there were about 5 instances in memory so I decided to use the killall command. It killed 2 processes and 3 were still left in memory. Tried again and the 3 were still there.

So I manually used the kill -9 command to kill each of the individual processes via their pids. That worked.

I mostly use the kill -9 command as it works. The killall command has let me down so many times I just don't bother using it. But there must be a reason why it doesn't work. Am I using it wrong?

I know there are other commands like pkill but I would be grateful to understand why the killall command doesn't work as expected. I have even tried to kill just one process and it is a hit and miss affair. But the kill -9 command works every time.

Sending signals to processes using kill on a Unix system is not a new
topic for most systems administrators, but I’ve been asked many times
about the difference between kill and kill -9.

Anytime you use kill on a process, you’re actually sending the process
a signal (in almost all situations – I’ll get into that soon).
Standard C applications have a header file that contains the steps
that the process should follow if it receives a particular signal. You
can get an entire list of the available signals on your system by
checking the man page for kill.

Consider a command like this:

kill 2563

This would send a signal called SIGTERM to the process. Once
the process receives the notice, a few different things can happen:

the process may stop immediately

the process may stop after a short delay after cleaning up resources

the process may keep running indefinitely

The application can determine what it wants to do once a
SIGTERM is received. While most applications will clean up their
resources and stop, some may not. An application may be configured to
do something completely different when a SIGTERM is received. Also, if
the application is in a bad state, such as waiting for disk I/O, it
may not be able to act on the signal that was sent.

Most system administrators will usually resort to the more abrupt
signal when an application doesn’t respond to a SIGTERM:

kill -9 2563

The -9 tells the kill command that you want to send
signal #9, which is called SIGKILL. With a name like that, it’s
obvious that this signal carries a little more weight.

Although SIGKILL is defined in the same signal header file as SIGTERM,
it cannot be ignored by the process. In fact, the process isn’t even
made aware of the SIGKILL signal since the signal goes straight to the
kernel init. At that point, init will stop the process. The process
never gets the opportunity to catch the signal and act on it.

However, the kernel may not be able to successfully kill the process
in some situations. If the process is waiting for network or disk I/O,
the kernel won’t be able to stop it. Zombie processes and processes
caught in an uninterruptible sleep cannot be stopped by the kernel,
either. A reboot is required to clear those processes from the system.

When you sent killall (SIGTERM) to the thunderbird processes, you requested those processes to stop. Some of those processes weren't working correctly (probably why you needed to kill them in the first place), so they couldn't act on the SIGTERM signal.